Skin Anatomy: Our skin is richly vascular; these vessels keep our skin healthy but also help regulate our temperature. Sensory nerves in our skin constantly feed information about our environment to the brain. Sensing cold, and we restrict the flow of blood to deeper vessels, protecting our core. The hotter our perceived environment and blood is diverted superficially to radiate heat. Our central nervous system also engages the sweat glands to spill off fluid—and with it—heat. Illustration by Steve Graepel

While on their way to the summit, the National Geographic Everest team will contend with many expeditious foils, including lack of appetite, decreasing atmospheric pressure, dubious weather, and no doubt the big chill.

But regardless if you are chopping steps at 8,000 meters or kicking glides on spring snow, the laws of thermodynamics universally apply. And understanding a bit how the body works can provide invaluable insight into how to layer your gear appropriately when chasing down your own summits.

Overheating: Above an ambient temperature of 88˚F, the body's thermostat kicks in its cooling system through sweating. But the hotter and more humid the environment around us, the more difficult it becomes to exchange heat...there's no gradient. A waterproof breathable shell requires a relatively high humid environment before moisture will pass through it and a thick fleece can trap sweat. As the temperature inside the shell increases, nerve sensors in the skin tell the brain "its getting hot - keep sweating". But sweating by itself does nothing in the way of cooling the body, so the cycle repeats itself. Illustration by Steve Graepel

Rules of Conduct(ion)
Thermal energy exchanges in a socialized economy. Always seeking a happy medium, particles with higher temperatures seek stasis by sharing energy with cooler particles. It’s called a thermal gradient, and its key to understanding how we cool.

This energy exchange is expressed as heat and it happens in four ways:

Conduction is the transfer of heat through contact with another solid object—like how your back feels cold when sleeping without a pad or your feet chill while standing on a frozen surface.

Convection is the transfer of heat through currents (like air or water). For example, when you ride or ski, the wind around you pulls heat away more quickly than if you were standing still. In general, the faster the current, the quicker the heat exchanges.

Radiation is the transfer of heat through wavelengths—like how on calm, crisp autumn mornings, your body feels increasingly colder as you stand still.

Lastly, evaporation is phase change of a liquid (sweat) to gas (vapor). We breathe out a significant amount of evaporative heat. As wind passes over the skin’s surface, the conversion of sweat to vapor pulls with it heat energy. We feel this cooling as wind-chill.

During an average day of pushing pixels, we’ll spend 60 percent of our calories pegging the thermostat at 98.6 degrees Fahrenheit—it’s our basal metabolic rate. Out of the office and into the saddle, we’ll rev up our caloric engines, burning off over 80 percent of our calories as heat, raising the core to about 100 ˚F—still relatively steady.

We owe a lot of our ability to regulate our temperature to water; it’s a very good insulator—in fact, it has the second highest specific heat capacity of all molecules (after ammonia). This means it takes a lot of energy to move its temperature up or down. As a result, it buffers large fluctuations in temperature, keeping our core temperature relatively steady. Good news for us, as we’ve got a lot riding on our ability to maintain our core temperature. A few degrees difference either way forces systemic decisions that sacrifice digits and kidneys to protect the heart and brain … which is a bad place to be.

To accommodate schizo-frenetic activity levels (like those experienced during high-altitude mountaineering) the body has developed several workarounds to micro-regulate our temperature, ensuring our core is always within an acceptable tolerance.

Hypothermia: Overheated, you have two options: stop or strip layers. Do both at once and you'll trip the thermal gradient from a cascade into a waterfall. Our hypothalamus is still telling the body to sweat to cool off as the outside elements rapidly whisk our heat away. This 'flash-off' effect can quickly push you into hypothermia. Illustration by Steve Graepel

More Than Skin Deep
Skin plays a key role in temperature regulation; it’s the consummate waterproof-breathable shell. Immediately under its surface is a layer of insulating sub-q fat, preventing heat from escaping too fast while buffering the external environment from freely robbing us of our heat. Furthermore, while our core temperature hums along at 98.6 ˚F, our skin runs cooler (86-88 ˚F). It’s our body’s internal thermal gradient, enabling a steady flow of heat to cascade from our core towards the skin.

Within the skin is an extensive network of sensory nerves—constantly sending feedback to the brain about our environment—and a micro-vascular plumbing system that runs both shallow and deep. When the brain senses temperatures under 86 ˚F, the body restricts circulation to its deeper vessels, hoarding warmth to protect the core. Above 88 ˚F and the body opens the flow of blood from the deep to the shallow vascular plexus and into a network of capillaries.

Now, when it comes to real estate, the body is like a Manila high-rise; a master of efficiency always looking for the most bang for its Peso. Throughout the body we see repetitive, plicated structures that exemplify this resourcefulness; we see it in cells (microvilli), the lining of our gut (villi), the structure of our brain (gyri), and in our skin. Between the dermis and epidermis, the dermal papillae folds into the epidermis (on your fingertips, you see these repeated as your fingerprint). In each papilla runs a mesh-work of capillaries, greatly increasing the vascular surface area, allowing conductive and radiant heat to quickly transfer to the outside world. It’s a bit like how your refrigerator radiates heat via cooling fins.

It’s no surprise that good cardiovascular health gives a person a leg up in regulating their internal temperature; something our Everest athletes have taken to heart. (Conrad Anker’s advice on staying in shape? Don’t get out of shape!)

We know water can hold a lot of heat. So wherever we store water … be it solid, liquid, our breath, our cells, or our sweat…with it we store heat. As we are exposed to hotter temperatures—either internally (a huge bi-product of burning calories is heat) or externally—the body triggers cells to leach water, which eventually collects in our sweat glands and is funneled to the skin’s surface, bringing with it heat.

Sweat by itself does nothing to help us cool. But move a current across the skin’s surface (like wind), and sweat will evaporate—exchanging heat energy with the surrounding environment—cooling the skin’s surface.

Equilibrium: The ideal solution is to wear a 'second skin' that continues to engage a thermal gradient between you and the outside elements, but provides enough buffer so that you are not subjected to immediate evaporative cooling and convective heat loss. Rather, moisture evaporates safely away from the skin, releasing heat at a consistent and comfortable rate. Illustration by Steve Graepel

Core Competencies
The hotter and more humid the environment around us, the more difficult it becomes to exchange heat; there’s no gradient for energy to move along. This is why overdressing in winter can feel like the Gulf Coast in August. Throw in abysmal weather, and the body’s safety net becomes porous … and this is generally where trouble starts.

Consider skinning up a backcountry peak wearing a fashion-forward, waterproof-breathable jacket. Skinning is hard work and quickly works up a sweat, but waterproof-breathable fabrics require high humidity before they can effectively transport moisture across its membrane. So as a humid microclimate builds inside the shell, heat is trapped. Like sitting in a Russian shvitz, the body’s thermostat kicks into overdrive, pouring out more sweat in a futile effort to cool the body. But it can’t. Sweat is trapped under a blanket of humid air, further insulating heat, signaling your body to sweat more to cool off … and the cycle repeats.

So while you are recklessly sweating out the contents of your CamelBak, you have two options: slow down (and get dropped) or strip down and get cold. Do both at once and you will immediately feel the super-cooling effects of evaporative heat loss—a flash-off —risking hypothermia … again, not a good place to be.

To stay comfortable while working, you need to mimic the body’s thermal gradient with a clothing system that gradually drops the temperature gradient from the skin’s surface to the outside. You need a microclimate.

This second skin starts with a lightweight, highly breathable base. Nothing new here; we all have a few pairs of these kicking around the bowels of our gear den and the crew on Everest will certainly be living in these. But how it works is worth noting. Its function is to lightly insulate (a less dense fabric allows air to work as an insulator) and quickly pull moisture away from the skin. By wicking sweat away from the skin, you reduce the effects of rapid conductive and evaporative heat loss. By spreading it across a larger surface volume, you allow evaporation to occur both readily and safely away from the skin.

Depending how much energy you are expending, a light insulation might be enough. Consider what the top finishers at the Boston marathon will be wearing … not much. But if you aren’t pulling down sub five-minute miles, a cold wind can cut right through this layer. So if you are feeling the chill, consider layering with a woven microfiber windshirt or jacket over the base. Microfiber is tight enough to block the convective cooling effects of wind and has some water repellency, but is porous enough for moisture to permeate across at a lower humidity than a waterproof-breathable. Some companies, like Montane and Marmot, offer a two-in-one solution: a wicking inner layer and a microfiber wind-block outer layer. Regardless, the results are the same; highly desirable thermal equilibrium. This combination can find a home in every kit: running, cycling, skiing, climbing … its simple, its cheap, its genius.

Keep in mind that there are several fabrics out there that are marketed as softshells, laminating a membrane for better overall weatherproofing. Unless you are flossing granite off-widths with your torso, these softshells can be overkill. While highly windproof and sporting better water repellency, they typically don’t breathe enough for on-the-go pursuits and can wet-out from sweat, slowing evaporative cooling; caught in a true deluge and they will soak. If you feel you need one, consider a woven fabric, which breathes better than a membrane softshell, while still providing protection from the elements (and Hoodoo cavity searches).

Does this eliminate the need for a traditional hardshell? No way. Wet and cold can be fatal, rapidly pulling heat from your core. So I’ll always bring something waterproof. But because I keep it in the pack 95 percent of the time, I look for the lightest, no-frills shell I can find. Unless you live in your gear, there is probably no reason to bring anything heavier than 11 oz. and many companies offer gossamer-weight shells tipping the scales between 6-8 oz.

If you expect to take extended breaks or spend the night out under a canopy of stars, you’ll want to pack some sort of insulation … either synthetic or down. Contrary to the trite adage (and applying what we know about water’s heat specificity), nothing is truly warm when wet. A wet garment—down or synthetic—will conduct heat energy from your core before reaching thermal equilibrium in its own boggy core. But a wet synthetic parka will hold its loft better than down (which will lose all its loft when wet), providing some space for your depleted warmth to settle in before it dissipates entirely.

In general, the more active or longer the pursuit, the more inclined I am to bring synthetic. When I stop, I’ll throw it on over my second skin to trap body heat. When I’m ready to roll, I’ll squeeze out the moist air in the jacket, pack it up and go.

Taking it to Extremes
I used to live in Minnesota. For a good time, we’d drive six hours to a crack in the North Shore wall where we’d take turns whacking ice picks into a frozen dribble. We’d also spend a lot of time fighting back waves of nausea as blood crept back into our frozen candy graspers. The screaming barfies—I miss’em already.

Twenty to one, the body prioritizes preserving its core warmth, often leaving the digits to fend for themselves. Sitting further from your core, extremities are more vulnerable to the elements. Everything that touches your hands and feet—even socks and gloves—pulls heat away…so happens an aluminum shaft conducts at a (much) quick rate. And that death grip on a pair of tools? It constricts vessels, hindering the flow of warming blood to the digits. Put a leash around your wrist and you restrict blood flowing into the hands even more so.

Furthermore, the wrists—like the neck, groin, and armpits—have prominent shallow vessels, creating ‘thermal windows’ that siphon heat off the core. Exposing these regions can leach serious BTUs. While not-so-good while ice climbing, pit zips and side zips can rapidly adjust your thermostat while on-the-go.

Lastly, different parts of the body have different surface to volume ratios. Do the math, and you’ll see that fingers, toes, arms, and ears have a greater surface to volume ratio than our trunks and legs. As a result, these regions lose heat much faster. This concept applies to smaller people, too—Emily Harrington has a greater surface to volume ratio than the other Everest team members and will be prone to cooling more quickly.

To help regulate your comfort zone, bring a variety of hats, gloves, and socks to match changing conditions.

_______

A coach once left me with “everyone is an experiment of one.” True, what works for one may not work for another. But the laws of physics aren’t biased and cold, wet and miserable is no time to pontificate on the principles of thermodynamics. And while most of us will never holiday working ropes on the Khumbu, staying comfortable is relevant at any altitude.

[…] This energy exchange is expressed as heat and it happens in four ways: Conduction is the transfer of heat through contact with another solid object—like how your back feels cold when sleeping without a pad or your feet chill while standing on a frozen surface. Outdoor Gear and Thermodynamics: How to Layer For Optimal Comfort … Even on Everest – Beyond the… […]

New Balance

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April 12, 2012, 10:11 am

Nice post, enjoyed the grafics even more since they illustrate how the body works. Hope to see more of this soon here at National Geographics. If one is looking for a nice stylish jacket i would recommend Khujo Jacken. They are nice to wear and mostly the girls like them.

Kayaker

April 20, 2012, 11:05 pm

Very informative and helpful article you have here. This made me realize that understanding the workings of our bodies more can help us take care of it.

[…] in the air thus allowing less oxygen to diffuse into your blood. The extreme environment also poses thermoregulatory and dietary problems for those attempting to climb. Base camp, at 17,600 feet, is high enough that […]